Positive opening and closing switch



June 1956 E. G. FRANKLEN POSITIVE OPENING AND CLOSING SWITCH 3 Sheets-Sheet 1 Filed NOV. 2, 1955 mow/9f 52 max/4,9770

50/10/70 6 [mm/(4M awa /0,?

" AZTO/P/VE) June 19, 1956 E. G. FRANKLIN 2,751,463

POSITIVE OPENING AND CLOSING SWITCH Filefl Nov. 2, 1953 5 Sheets-Sheet 2 5r /M/Z WM Arrow 5r June 19, 1956 E. G. FRANKLIN 2,751,463

POSITIVE OPENING AND CLOSING SWITCH Filed Nov. 2, 1953 5 Sheets-Sheet 3 Tlg. E

United States Patent PQSITIVE OPENENG AND CLOSING SWITCH Edmond G. Franklin, Minneapolis, Minn, assignor to General Mills, Inc., a corporation of Delaware Application November 2, 1953, Serial No. 389,627

11 Claims. (Cl. 200-137) This invention relates in general to switches and pertains more particularly to a switch especially suited for the thermoelectric control of appliances and the like.

One object of the invention is to provide a switch of the above general description which will have positive make and break characteristics without objectionable sacrifice in operational sensitivity.

More particularly, an aim of the invention contemplates a switch in which additional contact pressure is applied upon closing of the contacts and in which an accelerated separation occurs upon disengagement of the contacts, this dual feature being obtained with very little increase in cycling width.

Another object of the invention is to provide an incremental increase in contact pressure immediately following engagement of the contacts with a subsequent relaxation or reduction in said incremental pressure after a short period of time, such period being sufiicient to permit the switch actuating element, a bimetal in the most specific instance, to establish through its own action a 'firm degree of contact due to continued deflection thereof. Stated otherwise, provision is made for increasing the contact pressure during the early period of contact engagement, this being when the contacts would otherwise be vulnerable to rapid opening and closing with a concomitant deterioration of said contacts; substantial removal of the increased pressure is elfected once firm contact has been established by reason of the progressed deflection of the bimetal as above indicated.

A still further object of the invention is to provide a rapid opening or separation of the contacts immediately after the actuating element has permitted disengagement of the contacts, thereby avoiding any tendency for the contacts to flutter or chatter during this particular transitional period. Also, it is within the contemplation of the invention that once the contacts have become separated sufiiciently, then the contact gap will be of substantially the same magnitude that it would have been without the presence of my compensating feature.

Other objects will be in part obvious and in part pointed out more in detail hereinafter.

The invention accordingly consists in the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereafter set forth and the scope of the application which will be indicated in the appended claims.

in the drawings:

Figure 1 is a plan view of a preferred form of thermal switch according to the invention;

Fig. 2 is a side elevation of the switch of Figure 1, certain portions thereof being broken away along the directional line 2-2 of Figure 1, this view showing the parts at the moment of engagement of the contacts;

Fig. 3 is a fragmentary view corresponding generally to Fig. 2 but with the contacts closed for a comparatively short period;

Fig. 4 is the next successive operational representation.

the contacts being shown still closed but after a longer period of time has elapsed then that of Fig. 3;

Fig. 5 depicts the contacts just after separation thereof by the thermal actuating element;

Fig. 6 depicts the switch in a condition a comparatively short while after the contacts have been separated;

Fig. 7 presents an operational condition existing just prior to the closing once again of the contacts, the switch in this position resembling substantially the condition of the switch pictured in Fig. 2; and

Fig. 8 shows a theoretical contact displacement curve with the theoretical contact displacement plotted against time.

Referring now to the drawings, the unitary switch structure has been designated in its entirety by the reference numeral 10, the switch including a tubular supporting post 12 having a flange 14 at its lower end. Positioned superjacently the flange 14 is a bimetal element 16, the bimetal element serving in the illustrative embodiment as the actuating member for the contact arms presently to be described. A first contact arm 18 is held in spaced relation above the bimetal 16 and has in electrical contact with its upper surface a terminal 20. Spaced above the first contact arm 18 is a second contact arm 22 having a terminal 24 contacting its upper surface, the two contact arms being of resilient metal and normally biased toward each other. Interleaved between the bimetal 16 and the contact arms 18, 22 is a plurality of insulating spacers 26, the spacers serving as the means for electrically insulating these various members one from the other. For the purpose of supporting later described means for adjusting the bias of the contact arms 18 and 22, a bracket member 28 has one end positioned or encircled about the tubular post 12. In order to maintain all of the foregoing elements in a stacked or unitary relation, the upper end of the post 12 is riveted over or staked at 30. In this way a single bolt 32 may be employed in conjunction with a tapped recess 34 to hold the switch structure It) in juxtaposition relative to a device 36, the temperature of which is to be controlled. It is thought quite apparent that the device 36 may be the soleplate of an iron, the reverse side of a waffle baker grid, or any part of a member or structure which is to be heated in a regulated manner.

Presenting now a more detailed description of the contact arm 22, it will be observed that this arm carries a bridge strip designated generally by the numeral 38, the bridge strip resembling a single inverted letter W and which strip may also be considered to be a pair of inverted V-shaped struts, preferably stamped or punched from the contact arm 22 itself, thereby leaving a pair of parallel strips 40 and 42. Also forming a part of the bridge strip 38 is a central contact support 44. Each V-shaped strut accordingly includes a leg 46 connected to one end to the contact support 44 and a leg 48 connected at one end to the contact arm 22, these latter legs each being equipped with laterally extending wings or cars 50 to increase the mass of these particular legs 48 for a purpose hereinafter specifically described.

Since it is contemplated that current will course through both of the contact arms 18 and 22, a pair of co-operable contacts 52 and 54 is provided, the contact 52 being mounted at the free end of the arm 18 whereas the contact 54 is mounted directly on the contact support 44. Further, in the specific exemplified embodiment the bimetal 16 serves as the actuating agent for eflecting opening and closing of the contacts 52 and 54 and to carry out this intended purpose the bimetal 16 is terminally equipped with an insulating stud 56, the stud being disposed so that it engages the underside of the free end of the contact arm 22.

As hereinbefore indicated, it is contemplated that the biasing action of the contact arms 18 and 22 be adjustable for the purpose of determining the temperature at which their contacts 52 and 54 make and break. Accordingly, an adjusting shaft 58 is included as part of the switch 10, the shaft 58 comprising a knob 60 at its upper end which may be manually grasped for turning thereof, a threaded shank 62 engaging corresponding threads on the reversely bent end of the bracket 28, an insulating stud 64 disposed at the lower end of the shaft, a collar 66 circumscribing an intermediate portion of the shaft with a lug 68 extending radially so that engagement can be effected between said lug and a stop element 70 carried in an upright manner at the extreme end of the bracket 28. To permit engagement of the insulating stud 64 with the contact 18, there is provided an aperture 72 in the arm 22. Thus it will be seen that the switch structure is susceptible to use with a conventional and well known adjusting mechanism.

Assuming a typical operational cycle, reference should be first had to the position of the switch elements pictured in Figure 2, this figure showing the contacts 52 and 54 immediately after they have become engaged and current has begun to pass through them. To effect this condition of the switch it will be recognized that the member 36 has cooled sufficiently so that the bimetal 16 has at least partially straightened itself out and the movement of the bimetal toward the member 36 has permitted the contacts 52 and 54 to meet by reason of the fact that the contact arms 18 and 22 are biased toward each other due to the inherent resiliency thereof. Once current flows through the contact arms 18 and 22 via the contacts 52 and 54, inasmuch as the contact 54 is mounted on the contact support 44, the current thus flowing must pass through the bridge strip 38. Owing to the fact that the legs 46 and 48 of each V-shaped strut are approximately the same length initially and also due to the fact that the legs 48 are each equipped with the laterally extending wing portions 50 which serve to increase the mass of these particular legs, when current courses through the V-shaped struts, which it has to do in order to pass through the contact 54, then since the legs 46 are of lesser mass than the legs 48, the legs 46 will heat up more rapidly than the companion legs 48. Inasmuch as the legs 46 and 48 of each V-shaped strut are of relatively small cross section compared to the current intended to flow therethrough, it will be apparent that by virtue of the lesser mass of legs 46 these particular legs will expand lengthwise at a greater rate than the legs 48.

Passing now from Fig. 2 to Fig. 3 it can be seen in Fig. 3 that the increased rate of expansion of the legs 46 will result in a more pronounced lengthening of these legs than the other legs 48. Although, as will presently be made clear, the legs 48 will soon expand sufficiently to overcome or counteract the action produced by the legs 46, in the expanded condition pictured in Fig. 3 the legs 46 are instrumental in exerting an increased contact pressure between the contacts 54 and 52, this being true because the tendency for the contact 54 to move downwardly or normal to the general plane of the contact arm 22 is derived from the fact that the resultant motion is the difference between the upward expansion of the legs 48 and the downward expansion of the legs 46 and since at this particular moment the legs 46 have expanded more than the legs 58 there is a very pronounced tendency for the contact 54 to move downwardly. Whether the contact 54 actually moves downwardly depends on whether the contact arm 18 is of more resilient construction than that of the arm 22. In the pictured embodiment the contact arm 18 is more rigid than the arm 22 so actually there is an upward movement of the arm 22 which is evidenced by the amount of spacing shown in Fig. 3 between the free end of arm 22 and the upstanding stud 56 supported on the bimetal 16.

The condition of the switch elements illustrated in Fig. 3 is only a temporary one for once the legs 48 have had time to expand, then their reaching the same expanded state that the legs 46 have reached will result in a counteracting of the effect produced by the legs 46, as above alluded to when describing Fig. 3. Accordingly, it will be understood that Fig. 4 represents a stabilized condition of the bridge strip 38, the incremental pressure existing in Fig. 3 having been removed due to the legs 48, having reached substantially the same degree of expansion as the legs 46. Therefore, since the legs 46 and 48 are the same length, in Fig. 4 the incremental increase in contact pressure is virtually removed.

Now, supposing that the contact relationship illustrated in Fig. 4 exists for a sufiicient period to permit the member 36 to become more and more heated with the consequent result that the bimetal 16 Will become deflected more and more in upward direction, then it can be seen that the contacts 52 and 54 will eventually become separated by the action of the bimetal 16. This relation is shown in Fig. 5 where it will be observed that the contacts 52 and 54 are in a slightly separated condition, this condition having been produced by the upward movement of the insulating stud 56 mounted at the free end of the bimetal 16. Having become separated, the current through the contacts 52 and 54 ceases and once the current flow is interrupted contraction of the legs 46 will immediately start to occur, the contraction rate being more rapid than that of the legs 48 inasmuch as the legs 46 are of lesser mass than the legs 48. After a small amount of time has elapsed, the legs 46 will reach the same length that they originally assumed in Fig. 2, but since the legs 48 are contracting at a slower rate, there will be an upward relative movement of the contact 54 with respect to the general plane of its arm 22. Of course, after the legs 48 have had time to fully contract into the position shown in Fig. 7, then the resultant movement of the contact 54 is such as to be toward the contact 52 once again. However, by this time the heat emanating from the member 36 will be sufficient to cause enough deflection of the bimetal 16 so that the contraction of the legs 48 will not bring the contacts 52 and 54 together. In this way it will be recognized that immediately upon interruption of the current there is a very rapid contraction of the legs 46 which act to accelerate the movement of the contact 54 relative to the contact 52 and reliance is not made on just the deflective movement of the bimetal 16 to accomplish a positive separation of the contacts. The foregoing cycle is of course repeated once the member 36 has cooled sufficiently to permit contact re-establishment, and the cycle will begin over with the contact condition pictured in Fig. 2.

The advantages obtainable from the foregoing described structure are believed quite apparent. However as a further aid to a complete understanding reference may be had to Fig. 8 in which theoretical contact displacement is plotted against time. By theoretical contact displacement it is to be understood that actual contact displacement of the graph magniture would not ordinarily be obtained, for the displacement is actually restricted by virtue of the existence of the contact 52 which opposes any appreciable amount of movement of the contact 54 in a downward direction. Nevertheless upon the establishment of a flow of current through the contacts 52 and 54, the expansion of the legs 46 will create a tendency for the contact 54 to move downwardly and this increase in contact pressure is represented in the early part of the curve of Fig. 8. Of course, as the legs 48 become more and more heated, the initial effect of legs 46 is counteracted and the theoretical contact displacement in a downward direction tapers oi as shown in the first phase or segment of the pictured curve. When the current is interrupted by the opening of the contacts 52 and 54, the rapid rate of contraction of the legs 46 is instrumental in producing the central portion of the curve and it will be seen that this is substantially a replica, though inverted, of the curve resulting during the current on portion, a cooling action taking place in contradistinction to the earlier heating of the bridge 38. The third portion of the curve which is a current on condition once again is merely a repetition of the first current on" section of the curve and the curve repeats itself throughout the various cycles of operation.

In summation it should be emphasized that owing to the foregoing structural arrangement the contact pressure is increased during the early stages of contact and such increased pressure is automatically removed later on, thereby preserving the overall sensitivity of the switch. In other words, the contact pressure is increased when it is most needed without objectionably increasing the cycling width, and when the principles of the invention are pplied to a thermal switch, such as that exemplified, then the switch is capable of operating within a narrower range of temperatures than would otherwise be possible. Also, the desirable increase in contact pressure presents or paves the way for a second desirable end inasmuch as the separation of the contact is effectively accelerated upon interruption of the current, By virtue of the switch forming the subject matter of the instant invention, the life of the contacts 52 and 54 is prolonged for an indefinite length of time. Of necessity, the expansion of the struts and the degree of contact separation have been exaggerated somewhat for ease of recognition and comparison.

As many changes could be made in the above construction and many apparently widely diiferent embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. Y

It is also to be understood that the language used in the following claims is intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which as a matter of language might be said to fall therebetween.

I claim as my invention:

1. A contact structure for a thermal switch comprising an electrically conductive element, a contact, and means mounting said contact for movement relative to said element, said means including at least two current conducting struts having only their ends lying remote from said contact joined to said element, each strut being comprised of two portions inclined at an angle with respect to said element and capable of lengthwise expansion upon the passage of suflicient current therethrough with those portions nearest said contact having a more rapid rate of expansion than the other portions.

2. A contact arm subassembly for a thermal switch, said subassembly comprising an electrically conductive contact arm, a contact, and means mounting said contact for movement substantially normal to the general plane of said arm, said means including at least two current conducting struts having only their ends lying remote from said contact joined to said arm, each strut being comprised of a pair of lugs inclined at an angle with respect to the plane of said arm, all of said legs being capable of lengthwise expansion upon the passage of sufficient current therethrough with the leg of each pair nearest said contact having a more rapid rate of expansion than the more remote leg.

3. A contact arm subassembly for a thermal switch, said subassembly comprising a metallic contact arm, a contact and means mounting said contact for movement relative to the arm in a direction substantially normal to the general plane of said arm, said means including at least two current conducting struts each comprised of a pair of legs of substantially equal length inclined at an angle with respect to the plane of said arm, the total cross section of the struts being relatively small with respect to the current passing through the contact for causing substantial expansion and contraction of the struts by heating and cooling in response to passage and interruption of current through said contact, the leg of each strut which is farther from the contact being of greater mass than its companion leg adjacent said contact to thereby retard the rate of expansion of said farther legs are compared to that of said adjacent legs.

4. A thermal switch comprising a pair of co-operable contacts, a contact arm having a contact support on which one of said contacts is mounted, at least two V-shaped struts each comprised of a pair of inclined legs, the struts constituting the sole connection of the contact support to the contact arm and having a cross section limited with respect to the current passing through the contacts for causing substantial expansion and contraction of the struts by heating and cooling in response to the changes in current carried by the struts on engagement and disengagement of the contacts, the legs of said struts more remote from the contact support having portions thereof of greater mass to retard the expansion rate of these legs as compared with the expansion rate of the legs adjacent the contact support when the contacts engage to thereby increase contact pressure during the initial period of contact engagement.

5. A thermal switch comprising first and second contact arms having first and second contacts respectively adapted to engage and disengage each other, thermally responsive means engaging and moving one arm with respect to the other and thereby engaging and disengaging said contacts in response to predetermined temperature changes, the first arm having a contact support on which the first contact is mounted, and a pair of V-shaped struts, each comprised of a pair of inclined legs of substantially equal length, the struts constituting the sole connection of the contact support to the contact arm and having a crosssection limited with respect to the current passing through the contacts for causing substantial expansion and contraction of the struts by heating and cooling in response to the changes in current carried by the struts on engagement and disengagement of the contacts, the legs of said struts more remote from the contact support having wing portions with which to retard the expansion rate of these legs as compared with the expansion rate of the legs adjacent the contact support when the contacts engage to thereby increase contact pressure during the initial period of contact engagement.

6. A thermal switch comprising first and second contact arms, the first arm having a bridge strip resembling the letter W, a first contact carried by the central point of the W-shaped bridge and a second contact carried by said second arm respectively adapted to engage and disengage each other, the four legs of said W-shaped bridge being capable of lengthwise expansion upon the passage of sufficient current therethrough via said contacts with the two legs most remote from said contacts having laterally extending wing portions to provide a slower rate of expansion and contraction of these legs than that of those legs nearest said contacts, and thermally responsive means engaging and moving one arm with respect to the other and thereby engaging and disengaging said contacts in response to predetermined temperature changes, the expansion of said bridge due to current increase when the contacts engage thus increasing contact pressure initially due to the greater rate of expansion of the legs nearest said first contact, and the contraction of the bridge due to current decrease when the contacts disengage moving the first contact away from the second thus increasing the gap initially between the contacts due to the greater contraction rate of said legs nearest the first contact.

7. A contact structure for a thermal switch comprising a pair of cooperable contacts, an electrically conductive element, means mounting one of said contacts for movement relative to said element, said means including at least two current conducting struts having only their ends lying remote from said one contact joined to said element, each strut being comprised of two portions inclined at an angle with respect to said element and capable of lengthwise expansion upon the passage of sufficient current therethrough, a corresponding one of each of said two portions having a more rapid rate of expansion than the other in the general direction of the other contact.

8. A contact arm subassembly for a thermal switch comprising a pair of cooperable contacts, means supporting one of said contacts, an electrically conductive contact arm, means mounting the other contact for movement substantially normal to the general plane of said arm, said mounting means including at least two current conducting struts each comprised of a pair of legs inclined at an angle with respect to the plane of said arm, one leg of one pair together with the corresponding leg of the other pair converging in the direction of said first mentioned means and the remaining legs converging in a direction away from said first mentioned means, all of to the other and thereby engaging and disengaging said n contacts in response to predetermined temperature changes, the first arm having a contact support on which the first contact is mounted, and a pair of V-shaped struts, each comprised of a pair of inclined legs of substantially equal length, the struts constituting the sole connection of the contact support to the contact arm and having a cross-section limited with respect to the current passing through the contacts for causing substantial expansion and contraction of the struts by heating and cooling in response to the changes in current carried by the struts on engagement and disengagement of the contacts, those legs of the respective struts which converge toward a point in a direction away from said second contact arm having wing portions with which to retard the expansion rate of these legs as compared with the expansion rate of the remaining legs when the contacts engage to thereby increase contact pressure during the initial period of contact engagement.

10. A contact structure for a thermal switch comprising first and second separable contacts, an electrically conductive element, means mounting one of said contacts for movement relative to said element and relative to said second contact, said means including at least two current conducting struts having only their ends lying remote from said one contact joined to said element, each strut being comprised of two leg portions capable of lengthwise expansion upon the passage of sufiicient current therethrough, those leg portions adjacent said first contact diverging in the direction of contact separation and those leg portions more remote from said first contact converging in the direction of contact separation, said adjacent leg portions having a more rapid rate of expansion than said more remote leg portions.

11. A thermal switch comprising first and second cooperable contacts, a contact arm, means mounting said first contact for movement substantially normal to the general plane of the arm and relative to said second contact, said means including at least two V-shaped struts having only their ends lying remote from said first contact joined to said arm, each strut being comprised of a pair of inclined legs, the struts having a cross section limited with respect to the current passing through the contacts for causing substantial expansion and contraction of the struts by heating and cooling in response to the changes in current carried by the struts on engagement and disengagement of the contacts, the legs of said struts adjacent said first contact having a more rapid rate of expansion than those legs more remote from said first contact when the contacts engage to thereby increase contact pressure during the initial period of contact engagement.

References Cited in the file of this patent UNITED STATES PATENTS 2,289,131 Layton July 7, 1942 2,363,280 Arnold Nov. 21, 1944 2,388,033 Berninger Oct. 20, 1945 2,469,217 Sway May 3, 1949 

